A creature can grapple another creature. Characters typically grapple by using an Unarmed Strike. Many monsters have special attacks that allow them to quickly grapple prey. However a grapple is initiated, it follows these rules.
Grappled Condition. Successfully grappling a creature gives it the Grappled condition.
One Grapple per Hand. A creature must have a hand free to grapple another creature. Some stat blocks and game effects allow a creature to grapple using a tentacle, a maw, or another body part. Whatever part a grappler uses, it can grapple only one creature at a time with that part, and the grappler can’t use that part to target another creature unless it ends the grapple.
Ending a Grapple. A Grappled creature can use its action to make a Strength (Athletics) or Dexterity (Acrobatics) check against the grapple’s escape DC, ending the condition on itself on a success. The condition also ends if the grappler has the Incapacitated condition or if the distance between the Grappled target and the grappler exceeds the grapple’s range. In addition, the grappler can release the target at any time (no action required).
Grapple (an Unarmed Strike option). The target must succeed on a Strength or Dexterity saving throw (it chooses which), or it has the Grappled condition. The DC for the saving throw and any escape attempts equals 8 plus your Strength modifier and Proficiency Bonus. This grapple is possible only if the target is no more than one size larger than you and if you have a hand free to grab it.
SRD 5.2.1, p.182, also p.190 · CC BY 4.0